1. Field of the Invention
The present invention relates to a fully automatic passive rotational alignment system for the splicing of polarization-maintaining single mode fiber.
2. Description of the Prior Art
Polarization-maintaining single mode fibers present special difficulty in splicing or coupling. Ordinary single mode fibers must be aligned with each other laterally and longitudinally to within about 1 micron, and in angle to within a fraction of a degree. Polarization-maintaining fibers must also be aligned azimuthally; that is, they must be rotated relative to each other about their common axis until the fast and slow axes in their respective cores are also aligned. This is because it is essential for the successful application of these fibers that the transmitted light remain in the preferred polarized mode--either fast or slow--in crossing the splice. If the mode alignment is off by more than about 1 degree very serious losses take place. Not only does the projected power become divided between the two orthogonal modes of the receiving fiber, but the coupling into the originally-excited mode is very poor. An effective means of aligning such fibers is thus deafly desirable.
In U.S. Pat. No. 4,612,028, there is disclosed a polarization-preserving single mode fiber coupler made without mutually aligning the polarization axes of the fibers by twisting the fibers together over a selected length and fusing them. A critical requirement of this coupling method is that the initial misalignment be not close to 90 degrees.
As taught in U.S. Pat. Nos. 5,156,663 and 4,911,524, the principal manner of aligning polarization maintaining single mode fibers has heretofore been to rotate a first fiber relative to a second fiber while exciting the first fiber and monitoring the output from the second. That is, the first "transmitting" fiber must be aligned with a polarized light source for injection of light aligned with the preferred axis. Likewise, the output end of the "receiving" fiber must have its preferred axis aligned with a polarizing filter and detector. Thereafter, the ends of the fibers to be spliced or coupled are brought together in a suitable stage or housing, for instance on a fusion splicer. After the ends have been aligned laterally with each other in x, y, and z dimensions, to maximize the coupling of power across the gap, one fiber is rotated slowly relative to the other while the power received at the photodetector is monitored. Eventually an orientation is found at which the coupling of power into the preferred axis is optimum. The fusion or mechanical splice is then completed, by fixing the oriented ends together permanently.
Coupling of fibers using the method described herein above is subject to limitations. The two fibers must be coupled at their far ends with source and detector, and the transmitted power must be monitored continuously; the set-up required to power and monitor the fibers is difficult and time-consuming to establish; highly-skilled personnel are required; and the splicing procedure itself is time-consuming. If more than one pair of fibers is to be spliced the time and difficulty increase dramatically. There remains a need in the art for an improved method for coupling polarization-maintaining single mode fibers.